Sleep staging

Sleep staging is the classification of sleep into distinct stages — typically light, deep (slow-wave), and REM (rapid eye movement) sleep, plus wake — usually assigned in short epochs across the night, such as every 30 seconds. Each stage reflects a different pattern of brain activity, muscle tone, and eye movement, and cycles through several times over a typical night's sleep.

The clinical reference method is polysomnography, which combines brain-wave (EEG), eye-movement, and muscle-activity sensors in a sleep lab. Consumer wearables instead estimate sleep stage from indirect signals — motion, heart rate variability, and resting heart rate patterns — using a device-specific algorithm, and independent validation studies consistently find these estimates agree with polysomnography reasonably well on overall sleep versus wake, but are considerably less accurate at distinguishing the specific stages, particularly REM versus light sleep.

Consumer sleep-stage data is used within quantified self tracking to look at trends over time, and alongside training load and VO2 max as one input to recovery assessment, but a single night's staged breakdown from a wearable should be treated as a rough estimate, not a diagnostic reading. Anyone concerned about disrupted sleep, such as suspected sleep apnea or insomnia, should consult a qualified clinician rather than rely on consumer device output, which is not a medical device in most jurisdictions.